Impact of NH4OH treatment on the ion exchange and pore characteristics of a metakaolin-based geopolymer

We investigated the viability and influence of NH4OH post-synthetic treatment on the pore characteristics of geopolymers. Geopolymers are a class of materials with amorphous aluminosilicate three-dimensional frameworks, regarded as amorphous analogues of zeolites. Similar to zeolites, when geopolymers are used in catalysis or adsorption applications, post-synthetic treatments such as ion exchange with NH4+ salts (e.g., NH4Cl and NH4NO3) and desilication (using strong bases such as NaOH) are necessary to introduce active sites and modify their pore structure, respectively. Recently, it has been shown that treatment with NH4OH combines these two steps, in which acidic sites are introduced and the pore structures of zeolites are modified simultaneously. Considering the increasing interest in geopolymers in catalysis and adsorption applications, understanding the impact of such treatment on the structure of geopolymers is needed. Our diffuse reflectance infrared Fourier-transform spectra show that NH4+ exchanges Na+ in the geopolymer, and laser diffraction with scanning electron microscopy images show that the particle size of the powdered geopolymer decreases after NH4OH treatment. N2 sorption isotherms and 129Xe and 1H NMR measurements revealed information about the changes in pore structures: micropores were larger than mesopores and inborn mesopores increased in diameter, thereby reducing the surface area to volume ratio. However, pore accessibility and pore connectivity were not altered by NH4OH treatment. Since solid-state NMR and X-ray fluorescence revealed desilication, these changes in particle size and pore characteristics are considered to be due to desilication caused by NH4OH treatment.


129 Xe dynamic data and fits
The 129 Xe dynamic in site IP and BP were detected by measuring exchange rate (k), spin-lattice relaxation time (T1) and spin-spin relaxation time (T2).Selective-Inversion-Recovery (IR), 1 IR, 2 and Carr-Purcell-Meiboom-Gill (CPMG) 3 experiments were performed to get these three values, respectively.All of these data were analysed according to the following theories in MATLAB R2017b (Mathworks, Natick, Massachusetts, United States of America).
Based on the previous studies, 1,4,5 when the exchange process takes place between two sites, IP and BP, the time dependence of their magnetizations under 129 ere MIP(0) and MBP(0) are the magnetizations at t=0.
The 129 Xe T1IR signals intensities of peak IP and BP were derived by Topspin software (Bruker, Rheinstetten, Germany) and then fitted by the following equation, respectively, where M(t) is the magnetization at time t, Meq are the magnetization at equilibrium-state, T1 is the spin-lattice relaxation time and n is a free fitting parameter.
The fit equation for 129 Xe CPMG data is, where M(t) is the magnetization at time t, Meq are the magnetization at equilibrium-state, T2 is the spin-spin relaxation time.The signal intensity at each temperature step (S(X)) is dependent on the inverse temperature, X=1000/T.Their relationship agrees with the following equation, 6 where n is the number of phase transitions, S0i, Xci and σi are the signal intensity, the inverse transition temperature and the width of the temperature distribution curve of phase i, respectively.
Our signal intensity data as a function of X are shown in Fig. S3.The data were fitted with Eq.S5, assumed with containing three   where Tmelting is the melting temperature of bulk water, which is 273 K in our cases, k is a constant, 89 K*nm is used here.The k has the same value as our pervious geopolymer sample 7 we used to evaluate the K0 in Section S3, and it also agrees with the k value used for silica gel from one study. 8 The raw signals and fitted signals of three experiments are shown in Fig. S2.The detailed fit results of these signals are shown in Tables S1-3.

Figure S2 .
Figure S2.The raw signal (dots) and fitted signal (lines) of (a-c) 129 Xe selective IR, (d-f) T1IR and (g-i) CPMG experiments performed at room temperature of (a, d and g) N1, (b, e and h) N2 and (c, f and i) N3.
components (n=3), S, B and V, by using CFTOOL installed in MATLAB R2017b (Mathworks, Natick, Massachusetts, United States of America).The detailed fit results are shown in Tab.S5.

Figure S3 .
Figure S3.The raw (dots) signal intensity at each temperature step and the fitted (lines) signal intensity as a function of 1000/Temperature of (a) N1, (b) N2, (c) N3 and (d) N4.
Xe selective-IR experiment is given by,

Table S1 .
The fit results of 129 Xe selective-IR data.

Table S2 .
The fit results of 129 Xe T1IR data.

Table S3 .
The fit results of 129 Xe CPMG data.

Table S4 .
Fit results of cryoporometry data.